The invention pertains generally to the field of touch operated switch devices used to provide the interface between a human operator and data processing equipment, and has particular relevance to capacitive touch sensitive switch arrays that are employed for this purpose.
In present day practice, the most common manner of entering information into data processing equipment is through a mechanical keyboard composed of push button switches. These arrangements have numerous disadvantages normally associated with mechanical switches, such as requiring a large number of moving parts that are subject to wear, and exhibiting a relatively slow operation. In addition, since they are necessarily of an opaque and bulky construction, mechanical keyboards cannot be employed to interact directly with a computer display and their usefulness is accordingly limited.
A number of electronic computer interface systems have been developed which can be fabricated in a totally transparent or partially transparent form and which overcome many limitations of mechanical switches. These include optical, acoustical and capacitive systems. Optical and acoustical systems, which involve the interruption of light beams or acoustic waves by the finger, stylus or other such means, while being adaptable to interact with display screens of various forms and possessing a high information density capability, are relatively expensive and cumbersome structures. They also normally require a high degree of precision for their operation. For example, in the two more common systems of this type, the light pen and sonic digitizer, each require use of a hand held "pen" or stylus normally connected by cable to the electronics circuitry that must be positioned in a specific orientation to the light source or microphone and therefore requires a certain skill by the user. Maintenance and reliability of operation of sonic pen systems have also proven to be a major problem because of the precise alignment required of the microphones and their susceptibility to spurious noise signals. A precise alignment of the optical sensors is also required in the light pen systems. In addition, their use is limited to light scan type display screens and cannot be used, for example, with plasma panel displays.
Capacitive touch systems are primarily of two types. In one, the switch cells each comprise a single capacitor formed of two conductive plates, usually in the shape of interacting fingers, located on a single surface of a dielectric material. The capacitor is energized from an AC signal source and is coupled to a signal detection circuit, the detected signal being a function of the capacitance which is changed in response to touch. The switch cells of the other type of capacitive touch system each comprise one pair of series connected capacitors formed of three conductive plates, two closely spaced on one surface of a dielectric material and the third plate on the opposing surface overlapping the other two plates. In this device the series connected capacitors, which are energized by an AC signal source, form a voltage divider arrangement with the stray capacitance at the input to the signal detector. Touching the third plate alters the capacitive voltage divider arrangement, thereby changing the voltage level at the detection circuit as a function of touch. Capacitive touch systems are free of many of the problems noted with respect to the optical and acoustical systems, being of relatively inexpensive construction and readily operated through touch of a finger or stylus. However, they are generally limited by plate area requirements for obtaining a high density of switch cells in a given array. In addition, the numerous interconnections usually required for multiple switch cells make it difficult to achieve a totally transparent structure. Also, variable parasitic capacitances can adversely affect the operation of these systems.
The three plate device, to which the invention more specifically relates, is capable of good signal characteristics and an effective and reliable operation when sufficient signal is capacitively coupled through the series capacitor pair of each device. Within normal system parameters, this requires some minimum value of capacitance of the series capacitors, as well as relatively small parasitic capacitive effects. Since, for a given dielectric, the series capacitor capacitance is proportional to the capacitor plate area, presently known devices have minimum area requirements that preclude their application to arrays needing a large number of entry points within the constraint of a relatively small panel surface.
In addition, parasitic capacitive effects may often subject present day capacitive switch cells to false actuation caused by stimuli other than the actual touch of an operator. Several approaches have been employed to eliminate the problem which generally use shielding techniques. However, these are rather complex and costly solutions, and may not perform completely satisfactorily in all cases.
Present day array constructions, in addition to being restricted in the density of switch cells, normally require a discrete detection circuit for each switch cell. In some arrays which have a reduced number of detection circuits, a relatively complex scanning of the AC source voltage is required. In both types of prior art arrays, nevertheless, a large number of interconnections are employed which are undesirable for fabrication of the arrays.